Authorities in Australia have issued fresh warnings about a "mega blaze" after it spread beyond containment lines and razed 20 houses near Sydney.
The fire, burning over about 400,000 hectares, has moved further into the Blue Mountains - a popular tourist area which lies west of the city.
It is a blow to crews who are already battling over 100 fires and bracing for extreme temperatures this week.
The heatwave could bring the nation's hottest day on record, forecasters say.
Temperatures could exceed the 50.7C (123.26F) record set in Oodnadatta, South Australia, in 1960, according to the Bureau of Meteorology.
Since September, six people have died in a bushfire crisis that has engulfed the eastern states of New South Wales (NSW) and Queensland.
The blazes have destroyed more than 700 homes and blanketed towns and cities in smoke.
The bushfires and extreme weather has Australia's landscape, sparking public debate about the need for stronger climate action.
Last week, parts of Sydney suffered air quality 22 times worse than the clean air standard. Australia's largest city has about five million residents.
The smoke prompted an increase in hospital admissions and calls for a public health emergency to be declared.
What is the fire threat to Sydney?
Blazes are burning to the city's north-west and south-west. The mega blaze, also known as the Gospers Mountain fire, has encroached further in recent days.
Firefighters had tried to use cooler temperatures for "back-burning" - a tactic of deliberately starting small fires to remove vegetation before dangerous conditions arrive.
But in this instance the tactic backfired and crews lost control of the back-burn, said NSW Rural Fire Service deputy commissioner Rob Rogers.
About 20 homes were destroyed as a result amid 70m-high (229 ft) flames, he added.
"In these drought conditions, whatever we try doesn't seem to be working," Mr Rogers told the Australian Broadcasting Corporation on Monday.
The heatwave forecast for later this week will exacerbate those conditions. Across the nation, temperatures are set to exceed 40C in many areas.
Parts of Sydney could reach 46C by the end of the week, meteorologists say.